The present invention relates to cryogenic coolers and, in particular, to improvements therein for preventing temperature cycling.
Some cryogenic coolers, for example, sterling cycle, pressurized helium coolers, exhibit the undesirable phenomenon of temperature cycling above and/or below a specified temperature. A typical cooler includes a cold finger spaced from a dewar, with air in the space as a thermal conductor. Helium is used as the cooling medium in the cold finger. Detectors used in infrared imaging systems are secured to the dewar so that they can remain at a temperature of about 70.degree. K. Otherside, the detectors may improperly sense infrared radiation. Such coolers can cool down to a temperature of 60.degree. K., and then start temperature cycling with excursions which vary in excess of 40.degree. K. during periods in the order of one minute. Such temperature cycling is believed to be caused by a liquification of the air between the cold finger and the dewar. Because the most prevalent gas in air, nitrogen, liquifies at 77.degree. K., when the cooler temperature drops below 77.degree. K., liquid nitrogen forms on the dewar and results in a loss of gas which prevents heat transfer between the cold finger and the dewar. This loss of thermal conductivity allows the dewar temperature to rise to a point where the liquid again vaporizes, heat transfer resumes, the temperature drops until the gas again liquifies, and the cycle is repeated.
This temperature cycling is unacceptable and must be prevented. As stated above, cryogenic coolers are used to maintain the temperature of infrared detectors at a sufficiently uniform cryogenic temperature, so that the detection of infrared radiation will not be subject to error.